THE EFFECTS OF VOLCANIC ASH DISTURBANCES ON A PEAT-FORMING ENVIRONMENT - ENVIRONMENTAL DISRUPTION AND TAPHONOMIC CONSEQUENCES

The maceral and palynological composition of the C coal bed (Upper Cretaceous), central Utah, was significantly affected by the periodic deposition of volcanic ash in the precursor peat mire. The coal bed contains four altered volcanic ash partings (tonsteins). Sixty-seven coal and rock samples from 10 cores of the coal bed were examined for maceral composition, and 163 coal and rock samples from 10 cores were analyzed for palynological composition. Abundant semifusinite (an inertinite maceral with a poorly preserved cell structure and a light gray reflectance) is found in coal samples directly below the upper tonstein, the thickest (30-40 cm thick) of the tonstein partings. We suggest that (1) the pH of the water in the peat mire increased after deposition of the volcanic ash, creating an environment favorable to the formation of degradosemifusinite precursors (precursors to semifusinite that have a non-fire origin) by microbial activity, or that (2) chemicals from the volcanic ash, such as sulfuric acid, aided in the formation of a charred vegetation. We infer that deposition of the volcanic ash produced an environment conducive to the growth of ferns, as indicated by spores of pteridophytes that are found to be more abundant in samples directly above (in contact with) the upper tonstein than in samples directly below the upper tonstein. However, pteridophytes are also found in abundance in other areas of the coal bed; the growth of these pteridophytes is probably a result of hydrologic changes in the peat mire unrelated to volcanic activity. We suggest that leaching of the volcanic ash created a semi-impermeable layer, which caused the ponding of surface water. This hypothesis is supported by (1) elevated levels of the Sphagnaceae/Zygnemataceae group in shale or coal samples directly above tonsteins, (2) high levels of several macerals in shale samples directly above the upper tonstein: vitrodetrinite (a maceral defined as a fragment of a vitrinite maceral), inertodetrinite (a maceral defined as a fragment of an inertinite maceral), several liptinite macerals (sporinite, cutinite, and attrital resinite) that are concentrated during degradation due to their greater resistance to decay, bituminite (a liptinite maceral that may form as a decomposition product of organic material), and (3) abundant gelocollinite (a maceral defined as structureless groundmass material that formed from strongly decomposed plant parts) in coal samples directly above tonsteins in the coal bed.